This invention relates to a gas sensor and, more particularly, to a NOx gas sensor and a method and apparatus therefor, used for detecting the NOx gas concentration in exhaust gases of an internal combustion engine intended for industrial use and for driving an automobile, vessel or aircraft, or in combustion gases of a boiler or the like.
This invention also relates to a method and apparatus for measuring the composition of exhaust gases of a combustion device or an internal combustion engine, in particular the NOx gas concentration
This invention also relates to a method and apparatus for controlling a NOx gas sensor used for measuring the NOx gas concentration in an ambient atmosphere in which the oxygen concentration in a measurement gas is changed acutely.
This invention also relates to a method and apparatus for detecting the exhaust gas concentration for measuring the concentration of harmful gas components contained in the exhaust gas from the internal combustion engine and, more particularly, to a method and apparatus for calibrating the zero point of an output of the NOx gas sensor.
Recently, in dealing with an intensification of exhaust gas regulations, research has been constructed into controlling the operation of an engine or the catalyst based on direct measurement of the NOx in e.g., engine exhaust gases. The resulting NOx gas sensor, which uses an oxygen ion conductor, such as ZrO2, causes the measurement gas (detection gas) to be introduced into a first measurement chamber and a first oxygen ion pumping cell in the first measurement chamber pumps oxygen out, to an extent such as not to decompose NOx at all. Then, the gas lowered in oxygen concentration is introduced into a second measurement chamber where oxygen is further pumped out by a second oxygen ion pumping cell to cause NOx from the NOx-gas contained in the remaining gas to be decomposed. This decomposition is sensed as an electric current. In such NOx gas sensor, the NOx gas concentration is detected based on the current flowing across a pair of electrodes provided in the second oxygen ion pumping cell.
In general, a pair of electrodes are arranged on both sides of the oxygen ion conductor in such sensor. One of the electrodes is exposed to the atmosphere in the first measurement chamber, while the other electrode is exposed to a reference atmosphere containing oxygen. The electrical voltage applied across the first oxygen ion pumping cell is varied, based on an output of an oxygen partial pressure detection cell adapted for measuring the oxygen concentration in the first measurement chamber, in order to control the oxygen concentration in the first measurement chamber to a constant value.
However, various problems have been encountered in the course of investigations toward the present invention.
First, the output gain of the NOx gas sensor varies depending on the oxygen concentration upon measuring the NOx gas concentration, which results in an error in the NOx gas concentration measurement.
The present inventors have further found that, if the oxygen concentration in the measurement gas is varied significantly, delay is caused in voltage control for the first oxygen ion pumping cell, thereby making it difficult to measure the NOx gas concentration correctly. In particular, in lean burn engines using gasoline or a diesel fuel, which recently are used in increasing numbers, the oxygen concentration in the exhaust gas is varied significantly under the driving condition. Thus, if the PID control is performed based on the potential of an oxygen partial pressure detection cell it is difficult to correctly measure the NOx concentration in the exhaust gas if only the method of varying and controlling the pump capability of the first oxygen ion pumping cell is used.
In the above-described current limiting type sensor, one problem is that the detection current flowing in the oxygen ion pumping cell is as small as several xcexcA. Moreover, since decomposition of harmful gas components (such as NOx) is controlled or suppressed by the catalytic action on the electrode surface, another problem is that, on prolonged use, catalytic activity is changed causing a shift in the zero point (i.e., a detection output of the gas sensor specifying that the concentration of the pre-set component is substantially zero).
On the other hand, in an oxide semiconductor type gas sensor, which is based on the principle that changes in electrical resistance of an oxide semiconductor is proportional to the amount of a pre-set gas component adsorbed on the oxide semiconductor, suffers from poor reproducibility. Further, if moisture in the measurement gas is electrolyzed, causing it to be decomposed or dissociated, the in the second oxygen ion pumping cell, a current will flow due to oxygen yielded on electrolysis not only of NOx but also of moisture, resulting in an incorrect measurement of the NOx concentration. Moreover, in a lean burn engine using gasoline or a diesel fuel, the moisture in the exhaust gas is varied significantly under driving conditions. Consequently, it is difficult to make a correct measurement of the NOx concentration in the exhaust gases.
Thus, none of these types of gas sensor is reliably used as a sensor for a prolonged time and under significantly varying environmental conditions, as in the case of a sensor mounted on an internal combustion engine, in particular the sensor mounted on an exhaust gas system of a vehicle.
It is therefore an object of the present invention to provide a method and apparatus for accurately measuring NOx gas concentration in engine exhaust.
It is also an object of the present invention to provide a method and apparatus for controlling the NOx gas sensor which enables the NOx gas concentration to be measured accurately even if the oxygen partial pressure in the measurement gas is changed.
It is also an object of the present invention to provide a method and apparatus for measuring an exhaust gas concentration for a prolonged time and, in particular, a method and apparatus which enables calibrating a zero point of the detection output of an NOx gas sensor.
It is a further object of the present invention to provide a method and apparatus for calibrating a gas sensor in which the effect of the amount of moisture in the measurement gas on detection of the gas concentration is reduced or eliminated, in order to enable a correct measurement of gas concentration values.
Still further object of the present invention will become apparent in the entire disclosure.
According to the present invention one or more of the foregoing objects are achieved by several different features of the invention, based on one or more aspects of each feature as disclosed and claimed herein.
According to a first feature of the present invention, there is provided a measurement method and apparatus for measuring the NOx gas concentration in which the NOx gas concentration obtained as a NOx gas sensor output is corrected in response to the oxygen concentration in the detection gas (measurement gas). This is based on a correspondence between the gas sensor output and a current caused to flow by pumping out oxygen dissociated on decomposition of NOx.
An NOx gas sensor according to this first feature includes a first measurement chamber into which a detection gas is introduced via a first diffusion resistance, an oxygen concentration detection electrode for measuring the oxygen concentration in the detection gas in the first measurement chamber, and a first oxygen ion pump cell for pumping oxygen contained in the detection gas out from the first measurement chamber to the outside and/or inside of the first measurement chamber based on the potential of an oxygen concentration detection electrode. The pumping is generally to an extent such that NOx is not entirely decomposed, or in some case to an extent such that nitrogen oxide does not decompose into oxygen in the measurement gas. There also is a second measurement chamber into which the gas is introduced from the first measurement chamber via a second diffusion resistance, and a second oxygen ion pump cell having a pair of electrodes across which a voltage is applied to decompose the oxygen nitride in the second measurement chamber. The dissociated oxygen is pumped out, causing a current corresponding to the nitrogen oxide concentration (second oxygen pump current) to flow in the second oxygen ion pump cell.
According to an aspect of the first feature of the present invention, the NOx gas concentration can be found more correctly by a simple method even if the oxygen concentration in the detection gas does vary. Since the output of the oxygen concentration detection electrode has a specific relation with the oxygen concentration in the detection gas, both the oxygen concentration and the NOx gas concentration can be measured by a single NOx gas sensor, so that the NOx gas concentration, based on the oxygen concentration in the detection gas can be corrected using the sole sensor. Thus, the NOx gas concentration measurement apparatus according to the present invention can be applied to the exhaust system of the internal combustion engine in order to find the air-to-fuel ratio along with the NOx gas concentration, thus allowing a sole instrument to display multiple functions. The coefficient of the second oxygen pump current exhibiting oxygen concentration dependency, used for finding the NOx gas concentration, can easily be calculated preferably using the least square method. Thus, by pre-formulating a table correlating a coefficient with the oxygen concentration, the NOx gas concentration can be obtained correctly in real-time. If this method is applied to the NOx detection system for an internal combustion engine, it becomes possible to construct a highly flexible combustion control systems that can adapt itself to changes in the oxygen concentration and in the NOx gas concentration.
In a second aspect of the first feature of the present invention, the variation (varying amount) of the NOx gas concentration is a function of the variation (varying amount) of the second oxygen pump current. The coefficient of the variation of the second oxygen pump current in this function (xe2x80x98gainxe2x80x99) is varied in response to the oxygen concentration in the detection gas in order to find the NOx gas concentration.
In a third aspect, the gain as a coefficient of the variation (varying amount) of the second oxygen pump current is a function of the oxygen concentration in the detection gas. A detection gas having a known oxygen concentration and NOx gas concentration is pre-charged into a sensor to measure the second oxygen pump current. Using values of the NOx gas concentration and the second oxygen pump current, the variation of the second oxygen pump current to the variation of the NOx gas concentration at a predetermined oxygen concentration is found, e.g., by the least square method. Using the gain value at the predetermined oxygen concentration, the coefficient of the oxygen concentration as a function of the oxygen concentration in the detection gas and the gain is found, e.g., by the least square method.
In a fourth aspect the gain is represented as a function of the oxygen concentration in the detection gas. A detection gas having a known oxygen concentration and a known NOx gas concentration is pre-charged to measure the second oxygen pump current. Using the nitrogen oxide concentration and the second oxygen pump current, the variation of the second oxygen pump current to the variation of the NOx gas concentration (xe2x80x9cgain at a pre-set oxygen concentrationxe2x80x9d) is found, e.g., by the least square method. The gain at the pre-set oxygen concentration is selected and used depending on the oxygen concentration in the detection gas. *Note variation of NOx gas concentration divided by variation of the second oxygen current is defined as xe2x80x9cgainxe2x80x9d in this disclosure.
In a fifth aspect the gain is represented in terms of a logarithm function of the oxygen partial pressure in the detection gas. Preferably, a value of the oxygen partial pressure is used as the oxygen concentration in third aspect of the first feature of the invention.
In a sixth aspect, values of the second oxygen pump current for substantially zero NOx gas concentration and for a pre-set NOx concentration, obtained while varying the oxygen concentration in the detection gas, are measured. The gain at the preset oxygen concentration and the second oxygen pump current for the substantially zero NOx gas concentration for the preset oxygen concentration (hereinafter referred to as xe2x80x98offset at the pre-set oxygen concentrationxe2x80x99) are measured. From the gain and the offset corresponding to the oxygen concentration in the detection gas and the second oxygen pump current, the NOx gas concentration is found.
In a seventh aspect, the NOx gas concentration obtained based on the second oxygen pump current is corrected based on an output of the oxygen concentration detection electrode that is changed with the oxygen concentration in the detection gas.
In an eighth aspect, the variation (varying amount) of the NOx gas concentration is a function of the variation (varying amount) of the second oxygen pump current. There is provided gain selection means selecting the coefficient of the variation of the second oxygen pump current (hereinafter referred to as xe2x80x98gainxe2x80x99) in this function responsive to the output of the oxygen concentration detection electrode. There is also provided processing means calculating the NOx gas concentration based on the gain selected by the gain selection means based on the second oxygen pump current. The gain selection means can selectively use the coefficient of the gain as determined by, e.g., the least square method and the oxygen concentration in the deletion gas depending on an output of the oxygen concentration detection electrode. Alternatively, a previously found gain at a pre-set oxygen concentration can be selected depending on the output of the oxygen concentration detection electrode. Preferably, the gain selection means and the processing means may be constructed in a micro-computer connected to a nitrogen oxide concentration sensor.
Detection of the oxygen concentration can be based on the output of the oxygen concentration detection electrode means directly or indirectly detecting the oxygen concentration or the current or the voltage representing the oxygen concentration. Preferably, the oxygen concentration is detected based on the potential of the oxygen concentration detection electrode. Alternatively, the oxygen concentration is detected based on a voltage applied across the first oxygen ion pump cell controlled based on the output (potential) of the oxygen concentration detection electrode, and the first oxygen pump current flowing in the first oxygen ion pump cell. Thus, the gain of the nitrogen oxide concentration can be corrected based preferably on the voltage applied across the first oxygen ion pump cell and the first oxygen pump current.
Next, the aspects of the second feature of the present invention is hereinafter briefly explained. The aspects of the second feature is applicable with advantage to the NOx gas sensor to which the aspects of the first feature is applied with advantage. In the aspects of the second feature, the method for measuring the NOx gas concentration is comprised of several elements. That is, in a first aspect if the oxygen partial pressure (concentration) is changed, the detection output of the NOx gas concentration based on the second oxygen ion pump current is corrected in accordance with the change.
In this first aspect of the second feature, the NOx gas concentration can be measured accurately even if the oxygen concentration in the measurement gas would vary acutely. That is, even in an atmosphere in which the oxygen concentration varies significantly, the NOx gas concentration can be found correctly in real-time without control delay. By applying the control method of the aspects of the second feature to the nitrogen oxide detection system for an internal combustion engine, it becomes possible to construct a combustion control system which can adapt itself flexibly to changes in the oxygen concentration and in NOx gas concentration. The control device can be constructed both by software and by hardware.
In a second aspect, there is provided an oxygen partial pressure detection cell having an oxygen partial pressure detection electrode for detecting the oxygen partial pressure in the first measurement chamber. A detection output of the NOx gas concentration is corrected based on an output of the oxygen partial pressure detection cell.
In a third aspect, the voltage applied to the first oxygen ion pumping cell is controlled based on an output of the oxygen partial pressure detection cell. The detection output of the NOx gas concentration is calibrated based on the variation (varying amount) of the current flowing in the first oxygen ion pumping cell (referred to hereinafter as xe2x80x98first oxygen ion pump currentxe2x80x99).
In a fourth aspect, the voltage applied across the second oxygen ion pumping cell is controlled responsive to variations in the oxygen partial pressure in the measurement gas.
In a fifth aspect, if the oxygen partial pressure is low, the voltage applied across the second oxygen ion pumping cell is lowered. If the oxygen partial pressure is high, the voltage applied across the second oxygen ion pumping cell is raised.
In a sixth aspect, there is provided an oxygen partial pressure detection cell for detecting the oxygen partial pressure in the first measurement chamber or in the second measurement chamber. If, with the output of the oxygen partial pressure detection cell and the output of the second oxygen ion pumping cell as inputs, the output of the oxygen partial pressure detection cell is changed, a NOx gas concentration detection output based on the second oxygen ion pump current is corrected.
In a seventh aspect, there is provided an oxygen partial pressure controller for controlling the first oxygen ion pump current so that the output of the oxygen partial pressure detection cell will be constant. There is also provided a memory having the pre-stored relation between the variation of the output of the oxygen partial pressure detection cell and the of set of the second oxygen ion pump current. Responsive to the variation in the output of the oxygen partial pressure detection cell, pre-set data is read out from the memory. Based on the read-out data, the value of the offset of the second oxygen ion pump current is varied to correct the NOx gas concentration detection output.
The principles of a third feature of the present invention are identified with respect to the several aspects of this feature of the present invention. Specifically, the method for measuring the NOx gas concentration includes the following points: That is, in a first aspect of the third feature of the present invention, the zero-point of the detection output of the gas sensor, indicating the zero concentration of a specific component, is calibrated based on the detected output of the gas sensor in atmosphere. The concentration of the specific component is detected based on the calibrated detection output.
In this first aspect of the third feature of the present invention, the gas sensor is calibrated under a driving condition, among driving conditions of an internal combustion engine, in which the concentration of the component being detected is known or can be estimated to cancel the shift of the gas sensor detection output after prolonged use for assuring high precision detection of the concentration of the component under detection. The gas sensors capable of measuring the oxygen concentration include a HC sensor and a CO sensor in addition to the NOx sensor. In a sensor capable of measuring the oxygen concentration, it is possible to correct the sensitivity for the oxygen concentration by exploiting the fact that the oxygen concentration is not zero on fuel cutting but an atmospheric gas with an oxygen concentration of 20.9% is introduced. Also, since this calibration can be executed during fuel cutting or during the xe2x80x98richxe2x80x99 time in the air-to-fuel (A/F) ratio, it is unnecessary to set special operating conditions for executing the calibration. In particular, this calibration method is applied to a NOx gas sensor to enable correct measurement of the NOx gas concentration of the ppm order for prolonged time. Also there is a mode in which, if a NOx sensor for detecting the NOx concentration in the gas discharged from the internal combustion engine is provided downstream of the NOx occlusion type catalyst, the air-to-fuel ratio of a fuel rich region can be set for reducing the occluded NOx. This mode can be exploited to execute the above-mentioned calibration as well as to detect the state of deterioration of the NOx occlusion type catalyst. Further, the state of deterioration of the NOx detection reduction type catalyst arranged in an exhaust duct of the system employing a diesel engine for which the rich air-to-fuel ratio cannot be set, can also be detected.
In a second aspect of the third future of the present invention, the supply of fuel to the internal combustion engine is cut to set the concentration of the specific component in the gas introduced into the gas sensor as to be a level substantially equal to zero or to the atmosphere. Based on the detection output of the gas sensor on cutting the fuel supply, the zero point of the detection output of the gas sensor indicating the zero concentration of the specific component is calibrated.
In a third aspect, the rich air-to-fuel ratio of the internal combustion engine is set in a fuel rich side to reduce the specific component in the exhaust gas and to set the concentration of the specific component to be a level substantially equal to zero or to the atmosphere. The detection output is calibrated base on this level.
In a fourth aspect, the internal combustion engine is driven under a condition in which the concentration of the specific component in the gas discharged from the internal combustion engine can be estimated or is known. The detection output of the gas sensor is calibrated based on the detection output of the gas sensor under this operating condition.
In a fifth aspect, there is provided a gas sensor for detecting the concentration of a component of interest in the gas discharged from the internal combustion engine. There is also provided calibration means for calibrating the detection output of the gas sensor based on the detection output of the gas sensor under the operating conditions as set by driving condition setting means.
In a sixth aspect there is provided a NOx occlusion type catalyst in an exhaust duct of the internal combustion engine. There is also provided driving condition setting means for transiently setting the air-to-fuel ratio to be a fuel-rich side atmosphere for cleaning NOx occluded in the NOx occlusion type catalysts. There is also provided means for detecting the state of deterioration of the NOx occlusion type catalyst based on changes in the detection output of the NOx gas sensor before and after introducing the fuel-rich atmosphere.
In a seventh aspect, there is provided a NOx selective reduction type catalyst arranged in an exhaust duct of an internal combustion engine. There is provided a NOx sensor mounted on a downstream side of the NOx selection reduction type catalyst for detecting the NOx concentration in the exhaust gas. There is also provided means for adding HC to the exhaust gas in the internal combustion engine. There is also provided means for detecting the state of deterioration of the NOx selective reduction type catalyst based on changes in the detection output of the NOx sensor before and after HC addition.
In the exhaust gas concentration detection apparatus in an eighth aspect there is provided a gas sensor for detecting the concentration of a preset component in the gas discharged from the internal combustion engine. There is provided driving condition setting means for setting the driving condition of the internal combustion engine in which the concentration of a component of interest is known or can be estimated. There is also provided calibration means for calibrating the gas sensor detection output based on the gas sensor detection output under pre-set driving conditions.
In a ninth aspect, there is provided a NOx occlusion type catalyst arranged in the exhaust duct of the internal combustion engine. There is also provided a NOx sensor mounted downstream of the NOx occlusion type catalyst in the exhaust duct. There is also provided driving condition setting means for transiently setting the rich air-to-fuel ratio atmosphere for cleaning NOx occluded in the NOx occlusion type catalyst. There is also provided means for detecting the state of deterioration of the NOx occlusion type catalyst based on changes in the NOx sensor detection output before and after setting the rich atmosphere.
In a tenth aspect, there is provided a NOx selective reduction type catalyst in the exhaust duct of the internal combustion engine. There is provided a NOx sensor downstream of the NOx selection reduction type catalyst in the exhaust duct. There is provided means for adding HC into the exhaust gas of the internal combustion engine. There is also provided means for detecting the state of deterioration of the NOx selection reduction type catalyst based on changes in the detection output of the NOx sensor before and after addition of HC
According to aspects of a fourth feature of the present invention, there are certain occasions where the moisture provides more than a negligible influence on the sensor output.
In an engine, there is correlation between the excess air ratio or A/F value (=oxygen concentration in the exhaust gases) and the amount of the moisture in the exhaust gases, depending on the fuel used. The present inventors have found that, by pre-measuring the effect a influence of the moisture on the gas concentration detected by the gas sensor, storing the measured effect as a map in a memory as the relation between the excess air ratio or the A/F value, as an example, and correction data of concentration detected by the gas sensor, reading out pre-set correction data from the map during measurement depending on the excess air ratio or A/P value as determined by the driving conditions, and by correcting the detection output signal of the gas sensor based on the readout presumed amount of the moisture, the concentration of specific gas (such as NOx gas concentration) in the exhaust gases can be obtained accurately.
The present inventors have also found that, in the gas sensor of the type employing two sets of oxygen ion pump cells, the oxygen concentration in the exhaust gases can be measured from the electric current value of the first oxygen ion pump cell, and this oxygen concentration has correlation with the amount of moisture in the exhaust gas, so that, by pre-entering a correction coefficient associated with the electric current value flowing in the fast oxygen ion pump cell in a memory, preparing a map correlating the electric current value with the correction coefficient, and by reading out a specific correction coefficient in the map responsive to the electric current value of the first oxygen ion pump cell during gas concentration measurement to correct the electric current value of the second oxygen ion pump cell, correct gas concentration measurement free from the effect of moisture may be realized. The present inventors have also found that the output signal of the first oxygen ion pump cell can be electrically corrected on an analog circuit without having a map on the memory.
According to the several aspects of the fourth feature of the present invention, the gas sensor correcting method and apparatus is based on an estimate of the amount of the moisture in the exhaust gas based on from the engine operating conditions. The gas concentration detection signal outputted by the gas sensor is corrected depending on the estimated amount of the moisture. From the detected value of the gas concentration, the effect of the amount of moisture in the measurement gas is removed or reduced.
In a first aspect of the fourth feature of the present invention, the effect of the amount of the moisture in the detection (measurement) gas on the detection of the gas concentration may be diminished or removed to give an accurate gas concentration value. In measuring the concentration of a specified gas component in the exhaust gas discharged from an internal combustion engine, the amount of the moisture can be estimated from the engine operating conditions. In a gas sensor having two sets of oxygen ion pump cells, the amount of the moisture can be estimated from the electric current values flowing in the first oxygen ion pump current to enable output correction of the gas sensor responsive to the amount of the moisture in the exhaust gas by a relatively simple system. Moreover, by formulating the relation between the amount of the moisture or a value equivalent thereto and the gas sensor output in a map, reference can be made to this map responsive to the signal corresponding to the amount of the moisture or a value equivalent thereto to correct the gas concentration detection signal being output by the gas sensor based on the results of the reference to obtain more accurate gas concentration values.
In a second aspect, the gas sensor is a NOx sensor.
In a third aspect, the gas sensor is a sensor exploiting the oxygen ion pumping operation of the solid electrolyte.
In a fourth aspect, it is assumed that the ratio of the air to the fuel supplied to the engine (air-to-fuel ratio, A/F ratio) corresponds to the amount of the moisture in the exhaust gases. Based on the air-to-fuel ratio, gas concentration detection signal and the true gas concentration value, a map containing correction data of the gas concentration detection signal corresponding to the air-to-fuel ratio is pre-formulated. From this pre-formulated map, pre-set correction data corresponding to the air-to-fuel ratio as determined by the engine operating conditions are read out. The gas concentration detection signal is corrected by the read-out correction data.
In a fifth aspect, a specific or certain voltage is applied across the first oxygen ion pump cell to sufficiently pump oxygen in the measurement gas generally to an extent of not entirely decomposing NOx or in some case to an extent of not decomposing NOx. A current having a parameter corresponding to the oxygen concentration in the measurement gas flows in the first oxygen ion pump current. The second oxygen ion pump cell decomposes NOx in the gas left over in pumping to pump out dissociated oxygen. A current corresponding to the NOx gas concentration flows in the second oxygen ion pump cell. A NOx gas concentration detection signal corresponding to the electric current value obtained from the second oxygen ion pump cell is corrected responsive to a signal corresponding to the electric current value obtained from the first oxygen ion pump cell.
In a sixth aspect, it is assumed that the electric current value of the first oxygen ion pump cell corresponds to the amount of moisture in the measurement gas. Based on the electric current value of the first oxygen ion pump current, electric current value obtained from the second oxygen ion pump cell and the true NOx gas concentration, a map containing correction data of the NOx gas concentration detection signal (corresponding to the electric current value of each first oxygen ion pump cell) is pre-formulated. From this pre-formulated map, specific (preset) correction data corresponding to the electric current value of the first oxygen ion pump current is read out. The NOx gas concentration detection signal is corrected by the read-out correction data.
In a still further aspect, a residual gas is formed under the condition that allows and compensates for substantially decomposition of NO in the first flow channel or generally an upstream region in the flow channel.
This condition is established such that the in-flowing NOx from the ambient gas compensates the decomposed amount of NO to bring an equilibrium state in the residual gas.
The NOx is understood to be substantially the sum of NO and an amount of NO2 in an equilibrium determined generally by the temperature in which the rate of NO2 decreases as the temperature rises. For example, at room temperature it is almost NO2, at 300 to 400xc2x0 C. 50/50 and at 700xc2x0 C. or above NO2 is 5% or less.
The sensor is preferably operated at about 700xc2x0 C. or above and up to about 900xc2x0 C. more preferably at 750 to 850xc2x0 C.
Thus the role of NO2 in NOx is relatively small or negligible under the carefully operated conditions, or NO2 may be regarded as NO under certain condition which will be explained later.